1. Field of the Invention
The present invention relates to a device for controlling brakes of a vehicle such as an automobile, and more specifically, to such a device that controls braking force distribution among front and rear wheels in a vehicle.
2. Description of Prior Art
During braking of a vehicle, a rear wheel is liable to lock if its frictional circle shrinks due to a forward shifting of the load of the vehicle. The locking of a rear wheel, prior to locking of a front wheel, induces serious deterioration in vehicle running behavior, such as disturbance in the attitude and/or spinning of the vehicle body. In order to avoid the locking of a rear wheel, braking force distribution (BFD) control has been proposed to keep braking force on rear wheels lower than on the front wheels. In such BFD control, braking pressures applied to rear wheel cylinders in a hydraulic braking system are held, reduced or pulsatively increased, i.e. the increase in braking force generated on the rear wheels is restricted, providing a distribution of braking force biased to the front wheels. Usually, BFD control, often referred to as “Electronic Braking force Distribution Control (EBD control)”, is executed by a computerized device operating a plurality of solenoid valves in a hydraulic circuit. Examples of devices executing EBD control are seen in Japanese Laid-Open Patent Publications (JP) Nos. 5-213169 and 2001-219834.
Under EBD control, a demand from a driver of a vehicle for increasing braking force (e.g. a depression of a brake pedal) is modified to restrict a braking force on the rear wheels, which would cause the reduction of the total braking force. Thus, the driver would feel that the actually generated braking force is incompatible with his braking operation. In order to eliminate this feeling of the incompatibleness while maintaining the braking performance and behavior of a vehicle, JP No. 2001-219834 discloses an EBD control device, in which, after once restricted, braking force on rear wheels is pulsatively increased in response to the increase of a braking action by a driver under a certain running condition. To prevent locking of the rear wheels, however, braking force on the rear wheels cannot be increased limitlessly.
Accordingly, it is preferable that, in BFD control device, total braking force on a vehicle body may be rendered as close to the amount requested by a driver of the vehicle as possible, without inducing the locking of the rear wheels and the instability in the vehicle attitude induced therefrom.
Further, in the above-mentioned device, auxiliary braking control, such as Braking Assist Control (BAC) to be operated simultaneously with BFD or EBD control is not taken into account. Such auxiliary braking control increases a braking pressure beyond the amount requested from a driver of a vehicle so as to assist the driver in keeping her/his vehicle stable, so BFD control should be executed compatibly with any auxiliary braking control without reducing its effect.
Accordingly, a BFD control device may be improved more appropriately in conjunction with auxiliary braking control.